1. Field of the Invention
The present invention relates to a method of hiding data or patterns of a printed shadow image (ground tint of a document), particularly relating to a method of producing or detecting the hidden data or patterns of a printed shadow image.
2. Description of Related Art
With the rapid innovation of digitization, internet network and the advance of the duplication equipment, people can get the duplicated document much more conveniently than before. While people fully enjoy the modern information technology, the issues of the intellectual property right, the copyright management and document security also receive massive attention in recent years. The secured document includes banknotes, checks, identification card, a classified document and an exclusive particularity document of every organization, etc. The security actions, such as the anti-counterfeiting photo, the security printing, and the document authentication, often hide the information in a printed shadow image. The human eye is unable to perceive the hidden information, but it can be authenticated either by simple apparatuses or by machine-readable devices. It serves not only duplication preventing but also version control.
The anti-counterfeiting technology used on the present secured document often utilizes the special materials or the special methods of printing to achieve the anti-counterfeiting purpose, for instance, the fluorescent thread, the intaglio printing, micro prints, windowed security thread and so on. However, control of the special materials is costly and it makes the anti-counterfeiting technologies more and more important. The digital watermarking technology is used to hide (or embed) the hidden information or patterns in an image or a printed shadow images. This technology can authenticate the secure document by detecting the hidden information or pattern, even can trace the unauthorized duplication.
The technology using the microstructures, such as halftone dots, the fine screen line and the micro prints for anti-counterfeiting purposes has been around for a while. This kind of method does not need the use of special printing materials or a special printing method thereby reducing the cost of manufacture. The patterns of watermark are hidden in the microstructure of different sizes of halftone dot that exceeds the discernible ability of human visual system. The secure documents that use this technology to hide information or patterns cannot be distinguished by the human eye beyond a certain distance. However, the originally hidden information or patterns will be revealed after the duplicator makes the insufficient sampling of halftone-dot microstructures. This mechanism can avoid using a copy machine to produce a counterfeit or unauthorized copy.
Currently, there are some anti-counterfeiting technologies of a printed shadow image which using microstructure, for example, using different thickness of screen line, different sizes of halftone dots, different patterns or micro texts to construct the microstructure. The items below can be used for a basis of comparing advantages and shortcomings.                1. The invisibility of a hidden watermark.        2. The detection of the watermark by duplicating a printed shadow image.        3. The convenience and the variety of the watermark detection.        